Empty plastic crates may become brittle through exposure to sunlight, thereby possibly reducing their strength. Strength may also gradually diminish after long periods of use.
Empty crates can also be damaged in other ways. It is possible, for example, for the frame of an empty crate to develop hairline cracks or other cracks and holes or other damage. This can arise as a result of improper handling by consumers. Objects such as, for example, caps may also be left behind in the actual empty crate.
If a faulty crate were to be loaded with filled containers or bottles at the bottling plant, for example, and if the crate were to then break up, due for example to its existing weaknesses, there would be a significant production outage. A plant would come to a complete standstill and could only be returned to operation once the damage is rectified. For these and other reasons, empty crates should undergo an inspection before being filled with filled bottles or containers.
It is also possible for a crate to break while being handled by a consumer. In that case, the containers in the crate might then break on the floor, spilling product, including sugary drinks, on the floor. This involves extensive cleaning measures. It also adversely affects the reputation of the drink manufacturer, since consumers will most likely not bother to identify the crate manufacturer.
An inspection device can distinguish defect-free empty crates from faulty empty crates, and separate the faulty empty crates.
DE 33 00 259 discloses such a device for separating out damaged plastic crates using an ultrasound hardness tester with a vibration rod and a test probe at its head end. A drive device actuates the vibration rod in such a way that, in its pushed-in state, the test probe is in constant contact with the crate. A detector records the vibration rod's resonance frequency, which varies in the crate. Depending on the recorded resonance frequency, one can determined whether the crate is faulty or fault-free. If the crate is faulty it can then be separated out with a segregating device.
DE 103 21 389 A1 is also concerned with the defined excitation of a test specimen and recording the natural resonance, with small parts being examined for cracks and dimensional stability in this case.
DE 40 04 965 A1 is concerned with a leak testing of caps by creating and abruptly removing a magnetic field. In response, the cap rebounds and generates oscillation and/or the electrical signals. These signals, including frequencies thereof, are then processed and evaluated.
DE 74 07 378 U describes empty bottle inspection.
WO 03/45590 A1 discloses a method for detecting impurities and/or heavy materials in a mixture of waste or scrap by measuring the sound or structure-borne noise as compared with stored reference signals.
DE 2 310 869 relates to a test method for plastic-coated bottles.
A vibrometer, which is a measuring instrument for the quantifying of mechanical vibrations, is also known. Such vibrometers measure the vibration amplitude of an object.
Typically, a vibrometers has a laser that is focused on a surface whose vibrations are to be measured. As a result of the Doppler effect, which arises from the motion of the measured surface, the frequency of laser light that is back-scattered off the measured surface is shifted in frequency. This frequency shift is evaluated in the vibrometer by way of an interferometer and output as a voltage signal or digital data stream. A scanning vibrometer allows an area-related measurement of vibrations.
Laser scanning vibrometry is a fast method for the non-contact measurement and imaging of vibrations, for example in automotive, aeronautical and mechanical engineering, in microsystems and data engineering as well as in quality and production control. The optimization of vibration and acoustic behavior (e.g. operational vibration analysis) has become an important goal of product development in many of these fields because the dynamic and acoustic properties of products are some of their essential quality attributes.
In a scanning vibrometer, a laser Doppler vibrometer is integrated in a measuring head together with a scanner-mirror unit and video camera. During the measurement, the laser is scanned point-by-point over the test object's surface to provide a large number of very high spatial resolution measurements. This sequentially measured vibration data can be used to calculate and visualize animated deflection shapes in the relevant frequency bands from frequency domain analysis. Alternatively, it can be used to show vibration activity in the time domain, for example, by generating animations showing wave propagation across structures.
In contrast to contact measuring methods, the test object is essentially unaffected by the vibration measuring process. The measurement ranges accessible with known vibrometers completely cover the entire field of technically relevant vibrations. Vibrometry can thus be used to analyze, on the one hand, movements of microstructures with vibration displacements of a few picometers at frequencies up to 30 MHz (and hence v=0.1 m/s), and on the other fast-moving processes taking place in Formula 1 engines with vibration velocities up to 30 m/s.
A 3D scanning vibrometer combines three measuring heads, each of which detects dynamic movement from a different direction in space in order to completely determine the 3D vectors of motion. In the 3D representation of the vibration data, the vibrations of the measured object can be observed spatially or individually in the x, y and z directions, while in-plane and out-of-plane vector components can also be clearly differentiated.
With the known inspection devices that excite the empty crate that is to be inspected (e.g. DE 33 00 259), it is conceivable that, as a result of the standing up of the empty crate on the transport device, significant faults can corrupt the measurement result.